Method of making a shielded transformer



Aug. 24, 1965 J cox 3,201,854

METHOD OF MAKING A SHIELDED TRANSFORMER Original Filed Jan. 5. 1961 2Sheets-Sheet 1 w v 6 SQUARE WAVEGEN.

1 M f-9 mm 7 mm 5b C 5d VARAELE CHOPPER DEMODULATOR FILTER o.c..AMPLIFIER 2, g J0; J2) jQ) INVENTOR. Q Cbzy Aug. 24, 1965 J. A. COX

METHOD OF MAKING A SHIELDED TRANSFORMER 2 Sheets-Sheet 2 Original FiledJan. 3. 1961 N. mnEOIU 2 M W 0 ,7 We W \llrlj m a Q m v N a 4, -wm QR5925; J, u w w w Wm HFN QM w @R @1 \W r\ I]. .INL a N aw Q i-| I II I l3 I RQR \u A\% WW .ollgllll b 7 M l M02550 D M Q. M Wm" W MU (DOW W% uwQ 5 F United States Patent 3,201,854 METHOD OF MAKING A SHIELDEDTRANSFORMER Jay A. Cox, Rolling Hills Estates, Calif., assignor toGulton Industries, Inc., Metuchen, N.J., a corporation of New JerseyOriginal application Jan. 3, 1961, Ser. No. 80,467, now Patent No.3,149,296, dated Sept. 15, 1964. Divided and this application Sept. 7,1962, Ser. No. 221,965 1 Claim. (Cl. 29-15557) This application is adivision of application Serial No. 80,467, filed January 3, 1961, andnow Patent 3,149, 296.

The present invention relates to a method of making a sheldedtransformer which has particular utility in direct curent amplifierswhich are notably sensitive to pickup of unwanted signals which causenon-linearities or instabilities in the amplifiers.

One common form of direct current amplifier includes a chopper circuitwhich converts a direct current (DC) input signal to an alternatingcurrent (AC) signal having an amplitude proportional to the amplitude ofthe DC. input signal. The chopper circuit may include an invertercomprising two pairs of electronic switches, for example, in the form oftransistors or the like having control electrodes for rendering thedevices conductive or non-conductive depending upon the polarity andphase of control signals fed thereto. The two pairs of devices arerendered alternately conductive and are connected to an outputtransformer in a manner where the DC. input signal is alternately fed inopposite directions through the input or primary winding of the outputtransformer. The output transformer connects with the input of an AC.amplifier circuit whose output is coupled to a demodulator synchronizedwith the switching rate of the chopper circuit. The demodulator and anassociated filter network convert the AC. amplified signal to a filteredDC. signal.

The source of control signals for the electronic switches of the choppercircuit and the demodulator is preferably a square wave generatorproviding a number of separate output signals having a 180 phaserelationship. Signals having such a phase relationship are mostadvantageously obtained from the output windings of a transformer. Theinput and output windings of the transformer are wound in superimposedrelation on a core of magnetic material. Unwanted signals coupled byinduction between the square wave generator and the chopper circuit ordemodulator are readily eliminated by enclosing the latter transformerand, if necessary, other parts of the square wave generator, in aseparate housing constituting a magnetic shield. Unfortunately, however,prior to the present invention it was difficult to eliminate orsubstantially reduce unwanted signals capacitively coupled between thesuperimposed input and output windings of the latter-transformer whichsignals created unbalanced current components in the output of thechopper circuit.

and the demodulator which resulted in substantial nonlinearity orinstability in the D0. amplifier.

An object of the present invention is to provide an economical method ofmaking shielded transformers, most especially (but not necessarily)toroidal transformers,

which substantially eliminates said unwanted signals capacitivelycoupled between the input and output windings of the transformer.

The transformer construction to which the present invention has thegreatest utility has a conductive shield between the input and outputwindings wound around a core, the shield having a longitudinalinsulating gap which prevents the formation of a short circuit loop. Inaccordance with the present invention, the conductive shield is formedby coating as by spraying all exposed 3,201,854 Patented Aug. 24, 1965surfaces of the innermost windings, except for a longitudinal insulatinggap, with a conductive material, such as a low melting conductivematerial like zinc. The insulating gap is formed by applying a strip ofmasking tape around the outside perimeter of the partially wound corefor the full 360 thereof before the spraying or other coating applyingoperation. After the coating operation, the masking tape is removedleaving an annular insulating gap. The coating of zinc or otherconductive material is, of course, insulated from the main body of thewindings to be shielded as by the insulation surrounding the wireforming the windings.

The sensitivity of many D.C. amplifiers is such that the aforesaidinsulating gap is, in many cases, of sufiicient size to allow passagetherethrough of a significant interfering electric field which wouldadversely effect the operation of the amplifier. The present inventionovercomes this difiiculty by separately shielding the insulating gap inaway which avoids the formation of a conductive bridge across the spacedlongitudinal margins of the conductive coating bordering the insulatinggap. This is most advantageously accomplished by first applying a stripof insulating material over the insulating gap, the insulating materialbeing sufiiciently wide to extend beyond the margins of the gap. Anarrower strip of conductive material is positioned within the marginsof the strip of insulating material so as to shield or cover theinsulating gap. Both the coating of conductive material and the strip ofconductive material covering the insulating gap are electricallyconnected to ground or other common reference voltage point as by meansof a single bare ended conductor soldered between the strip ofconductive material and only one of the longitudinal marginal portionsof the conductive coating bordering the insulating gap. The strip ofconductive material is thus isolated from direct electrical contact withthe other longitudinal marginal portion of the conductive coatingbordering the insulating gap, to prevent the formation of a conductiveloop. The winding or windings which are to be shielded from the innerwinding or windings of the transformer unit are then wound over theshielding structure just described.

Other objects, advantages and features of the invention will becomeapparent upon making reference to the specification to follow, the claimand the drawings wherein:

FIG. 1 is a simplified box diagram of a DC. amplifier in which thepresent invention has particular utility;

FIG. 2 is a circuit diagram of a part of the DC. amplifier shown in FIG.1;

FIGS. 3 and 4 are perspective views showing successive stages in theprocess of fabricating the shielded transformer forming part of the DC.amplifier of FIGS. 1 and 2;

FIG. 5 is an enlarged fragmentary view of the partially made transformerof FIG. 4;

FIG. 6 is a fragmentary broken away view of a completed transformerconstructed in accordance with the present invention;

FIG. 7 is a transverse section through the transformer of FIG. 6, takensubstantially along the section line 7-7 therein; and

FIG. 8 is a plan view of a completed transformer con- I 9 v renderedalternately conductive under control of a source of signal voltage fedfrom a square wave generator 6. The

square wave generator has an' output transformer 8 with.

at least one primary or inputwinding 8t; and a 'seriesof secondary oroutput windings 8b, 8c and 8d wound 'on a i saturable core 8. Theconnections made between the output windings 8b and 8c and the choppercircuit are such that the voltages are applied to the choppercircuitfrom these windings 180 out'of phase. The customary chopper circuit usedin DC amplifier circuits requires that these connections all beungrounded, that is floating with respect to ground. In thisenvironment, the problem 7 of capacitive coupling of signals from theinput winding 8a to the chopper circuit via the output windings 8b and80 becomes so significant that they can very seriously adversely affectthe operation ofthe D.C. amplifier system.

The invention provides a unique electrostatic shielding construction 9diagrammatically illustrated in FIG. 1

which minimizes or eliminates this capacitive coupling.

The A.C. output of'the chopper circuit 4 is fed to a DC.

ampifier 10 and then to a demodulator circuit 12 which converts theA.C.voltage'to a pulsating direct current voltage. In the manner to bedescribed, the demodulator is operated in synchronism with the choppercircuit by means of floating connections from the outputwinding deredconductive and non-conluctiveby the feeding of suitably phased voltageto the base electrodes'41 and 43 of transistors Tl and T2 and baseelectrodes 45 and 47 of transistors T3 and T4. The upper terminal ofoutput winding 8b of .the square Wave generator transformer 8 is coupledby'a conductor 50 'to a resistor 52 connected to the baseelectrode 41 oftransistor T1. The upper terminal of the output winding 80 is coupled byconductor 54 to a resistor 56 connected to the base electrode 43 oftransistor T2... The windings 8b and 80 have center tapped pointsrespectively connected by conductors 57 and 59 to the commonly connectedcollector, electrodes of transistor pairs T1"T3 and T4T2. It-is thusapparent that the'phaseof the induced voltage at the upper termimale ofoutput windings 8b and 8c is identical and that .;the transistors ,T1and T2 are simultaneously rendered conductive and'non-conductive duringsuccessive half cycles of the square wave output'of the transformer 8.

8d ofthe transformer 8 to the demodulator circuit; The

pulsating DC. signal is then filtered by a suitable filter circuit 14 toprovide the resulting amplified DC. signal.

' The specific nature of the DC. amplifier system can, 7 v

of course, be varied widely and the components thereof I just describedcan be any one of a number of well known 7 types. For purposes ofillustration only, exemplary circuit details for the chopper, squarewave generator, demodulator and filter circuits shownin FIG. 1-are'illus tratedinFIG.2.. K V I V The chopper. circuit as illustratedincludes a first pair of PNP transistors T1 and T2 and a second' pair ofPNP transistors, T3 and T4. The collector electrodes 16 and I 18 of thetransistors T1 and T3 are connected by a conductor 20 to the negativeterminal .22 ofthe source of variable DC signal voltage 2. The collectorelectrodes-21 and The bottom terminal of output winding 8b is coupled bya conductor 58 to a resistor 60 connected to the base electrode 45 oftransistor T3. The bottom terminal of has a center tap point 73connected to the" positive terconnected to 'a ground conductor ,80. The.ground con minal of a source of direct current voltage 74, the negativeterminal of whichis grounded. The transistors T5 and T6 haveemitterelectrodes 76 and 78 respectively ductor extends to' the upper. terminalof a feedback or control winding 8e wound on the core 8' of thetransformer unit '8. The bottom terminal of the'winding8e isconnectedthrougha resistor 83 of'the base electrode 23 of thetransistors T2 and T4 are connected through a conductor 24' to thepositive terminal 26 of the variable D.C.,signal source. "The emitterelectrodes 28'and 30 of transistors T 1 and T4 are connected together bya con ductor and, the emitter electrodes32 and ,34 of the transistors T3and T2 are connected together by a' conduct0r35. The latter conductor 35is connected by a conductor 36 to'o'ne end of the input-winding 38a ofan output transformer 38. The conductor 31 connecting theemitterrelectrodes 28' and'30 of the transistors T1 and T4 are connectedthrough a conductor 40-tothe other end 'of the input winding 38a, r

As will appear, when the first pair of transistors T1 and T2 arerendered conductive, the path forcurrent flow through the choppercircuit from'thev negative terminal 85 of the'transistorTS. The groundconductor 80 also extends to thebottom terminal of a second feedback orcontrol winding 8 whose upper terminal is connected through aresistor 87to the base electrode'89 of the transistor T6. A capacitor-resistornetwork 87 is connected between'the collector electrodeg6 5oftransistorTS and 220i the variable D.C. signal source 2 can be tracedI through the conductor 20, collector and emitter electrodes 16and 28 0fthe transistor 1, conductors 31 and 40, the inp'ut'winding 38a in adirection from the bottom to the top terminals thereof, conductor 35,the emitter and collector electrodes 34 and 21 of transistor T2, and

conductor'24ileading to the positiveterminal'26'of the variable'DC.signal source.

When the second pair of transistors T and T4 are conductive, currentflow can be traced in a path extendingfrom thefnegative'terminal 22through the collector andernitter electrodes 18 and 32 0f transistor T3;conductor 36 leading tothe upper end of p 7 the inputwinding'38a,conductor 40, emitter and collector electrodes 30 and 23 of transistorT4 and the conductor 24 leading to the positive terminal 26.; I

the base electrode 89 ofthe transistor T6. A similar capacitor-resistornetwork 89'isconnected b'etweenthe collector'electrode v67 of thetransistor T6 and the base electrode 85 of the transistor T5. Thesefeedback networks aid in reducingthe changesover timewhen the conductivecondition of the transistorsT5and T6i reverse. When one of thetransistors T5 initially becomes conductive,

' the resulting'flow offcurrent through the input winding 8a generates afeedback voltage in the feedback winding 8e which maintains theconduction of the'transistor T5. Conversely, the voltagewinduced in theother feedback ,winding8f at that instant is in a'direc'tion which keepsfthe transistor T6 non-conductive.

transformer unit 8 ismade or a rectangular 'hysteresis The. core 8f ofthe materialfandwhen this material saturates, the senseof "the voltagesthen induced in the feedback windings 8e and 8f reverses'totriggerthe'then non-conductive transistorf i'nto aconductive state andthe conductive, tranrsistor into a non-conductive state. It can be shownthat the output voltage induced 'infthe' output" windings 8b,-

8c and 8d is substantially in FIG. '2.

a square wave as illustrated As. previously indicated,- the. choppercircuit .4 provides a fiow'of alternating eurrentin the input winding38a i As previously indicated, the meansfor' opening and closing theelectronic switches formed by the transistor devices' T1T2 and T3-T4includes control signals'fronr the'square wave generator 6.-- Thetransistors are ren-f {of the output'transformer 38 whose amplitude isproportional to the amplitude "of the input 'D.C. signal. voltage fedfrom the'source2. Transformer 38 has an output winding'38b feeding theinput ofuan',A.C. amplifier 10 terminals 9496.

' time.

which may be a conventional type amplifier. The amplifier has an outputtransformer 90 with an input winding 90a and an output winding 9% whichfeeds the input of the demodulator circuit 12.

The demodulator circuit includes a pair of rectifier bridge networks 92and 92'. The bridge network 92 includes a first pair of rectifiers 92aand 92b connected in series in the same sense between a pair of oppositebridge It also has a second pair of rectifiers 92c and 92d which areconnected in series in the same manner between the terminals 94 and 96.

The other bridge network 92' comprises a pair of rectifiers 92a and 92bconnected between terminals 94' and 96' but arranged in the oppositesense to the corresponding rectifiers 92a and 92b in the other bridgenetwork 92 so that the path for current flow is between terminals 94'and 96 instead of between 96 and 94. The second bridge network includesa second pair of rectifiers 92c and 92d which are connected in series inthe same sense as rectifiers 92a and 92b between the terminals 94' and96'.

The bridge network terminals 94 and 94' are con nected throughrespective resistors 96 and 96' to a common conductor 98 extending tothe bottom terminal of the output winding 8d of the square wavegenerator transformer 8. The bridge network terminals 96 and 96' areconnected through respective resistors 100 and 100' to a commonconductor 102 extending to the upper terminal of the transformer outputwinding 8d.

The upper terminal of the amplifier output transformer winding 9% isconnected by a conductor 104 to the juncture between rectifiers 92a and92b of bridge network 92 and the bottom terminal of the latter windingis connected by a conductor 106 to the juncture between the rectifiers92c and 92d of the bridge network 92.

The juncture between the other pairs of diodes 92c-92d and 92c'92d' ofthe two bridge networks are connected to a common conductor 107extending to one of the inputs of the filter network 14. The amplifieroutput transformer winding 90b has a center tap point which is connectedby a conductor 109 to the other input of the filter network 14. Theinput conductor 109 extends to a series circuit of a resistor 111, afilter choke 113 and a filter choke 115 leading to an output terminal117 of the filter network. The other input conductor 107 to the filternetwork extends to the other output terminal 119 of the filter network.Filter capacitors 121 and 123 are connected between the opposite sidesof the filter choke 115 and the input conductor 107.

It is apparent that the frequency of the signal in the amplifier outputtransformer winding 90b and the control signal fed to the demodulatorcircuit from the square wave generator transformer winding 8d isidentical, the amplitude of the former signal varying with the amplitudeof the variable input DC. signal and the output of the latter signalbeing constant. The polarity of the alternating current signals fed fromthese two sources to the demodulator circuit also change at the sameinstant of It can be shown that the demodulator circuit just describedis so designed that the alternating current output from the transformer90 is converted to a constant DC. signal at the output of the filternetwork 14 having an amplitude proportional to that of the variable D.C.input signal delivered by the signal source 2.

It can be appreciated that the useful signals coupled between theprimary winding 80 of the square wave generator transformer 8 and theoutput windings 8b, 8c and 8d are inductively rather than capacitivelycoupled. Un- Wanted signals inductively coupled to the chopper circuitand demodulator circuit can be avoided by enclosing the square wavegenerator in a separate housing made of magnetic shielding material. Anysignals Which are capacitively coupled between the input and the outputwindings of the transformer would also adversely affect the DC.amplifier by creating unbalanced current components in the system whichwould result in instabilities or system.

Refer now to FIGS. 3 through 8 which show the construction of the squarewave generator transformer 8. The transformer has a toroidal core 8'made of a rectangular hysteresis core material. The input winding 8a maycomprise a wire 8a having a suitable covering or coating of insulation8a" as in the case of conventional insulated wire used in thefabrication of transformer windings. The insulated wire 8a is woundaround the core 8' in a conventional way and may constitute one or morelayers of wire turns extending part way around or completely around thetoroidal core. The feedback windings 8e and 8 may, if desired, occupy aposition around or beneath the turns constituting the input windings 8aor they may be wound around different segments of the toroidal core 8'not occupied by the input winding 8a, where the latter does not extend afull 360, These details, of course, have nothing whatever to do with thepresent'invention.

The shielding 9 between the input and output Windings of the transformerincludes a coating 124 of electrically conductive material applied overthe innermost of these windings, the input windin g 8a in the exemplaryform of the invention being described (and the other windings 8e and 8fwhere they constitute inner windings of the core along with windings8a). The conductive coating, most advantageously, is zinc sprayed inmolten form over the entire exposed surface area of the core unit beforethe output windings 8b, 8c and 8d are applied, except for a peripheralannular insulation gap 126 extending all the way around the core unit.The insulation gap 126 prevents the formation of a short circuit loopwhich would adversely effect the operation of the transformer. Theinstallation gap 126 is most advantageously formed in the mannerillustrated in FIG. 3. Before the molten zinc coating is sprayed on thecore unit, a strip 128 of masking tape is secured around the outside ofthe partially wound core unit. Also, prior to the application of themolten zinc, a winding of Mylar or similar insulation is wound aroundthe partially wound core unit to protect the insulating coating 8a",etc. of the subjacent winding or windings from the hot zinc which coulddestroy the coating. The winding 125 can be omitted where the insulation8a" is not adversely affected by the application of the coating 124.Then the entire exposed surface of the core unit is sprayed with zincand the masking tape 128 is then stripped from the core to leave thecontinuous insulating gap 126. In one embodiment of the invention, theinsulating gap had a width of A of an inch. However, the exact width ofthe insulating gap is unimportant. Zinc is the preferable material forthe conductive coating 124 since it has high conductivity and a lowmelting temperature which will not harm or destroy the masking tape 128or other insulation materials beneath the coating,

Despite the fact that the insulating gap 126 occupies only a smallfraction of the area covered by the conductive coating 124, it has beenfound that for DC. amplifier applications the insulating gap 126described above provides a sufiicient space that capacitive coupling tothe output windings 8b, 8c and 8d is significant, particularly insituations requiring severe operating requirements for the DO.amplifier. To prevent such undesired capacitive coupling, the insulatinggap 126 is covered by conductive material in a manner which does notbridge the longitudinal marginal portions of the conductive coating 124bordering the insulating gap. This is accomplished by first applyingaround the entire core a strip 130 of insulation material ofsubstantially greater width than the insulating gap 126 so that thelongitudinal margins thereof extend well beyond the gap, as shown mostclearly in FIGS. 6 and 7. The strip of insulating material may be madeof Myler insulation having an adductor 142,

f 7 hesive coating onthe inner side to the conductive coating 12,4.

. A- strip of conductive material 136 of tinfoil or the for adhering thesame] like is adhesively or otherwise applied over the strip ofinsulating material 130 for'the fullt360": of the toroidalcore unit. Theconductive stript136 is somewhat wider than the insulating gap 126 so asto'extend beyond the longitudinal margins thereof, but is narrower thanthe;

strip of insulating material 130 sothat it islocated completely withinthe longitudinal margins thereof" 1 The conductive strip 136 iselectrically connected to the conductive coating 124 by means preferablyincluding thebared wire end portion 146 ofan insulated C011?circuinferentially around the outer portion of the core unit as shown inFIG; 7 and is soldered or otherwise electrically and physically anchoredbetween the conduc,

tive strip 130 and the conductive coating 124; The bared wirete'ndportion 140 is thus secured ,to'only one ofthe longitudinal marginalportions of the conductive coating bordering the insulatingrgap 126, sothat the baredwire quickly and easily applied so that the transformerscan be; mass produced. The spraying of the zinc coating 124 is ofparticular value inthis regard, although the broader aspects of the"invention envision theapplication of the, 1 coating 124jbyother means.7 I a p Various additional modifications may be" madein the,

transformer described" above without deviating from the broader aspectsof the invention.

l What I claim as new and desire to; protect by .Letters Patent of theUnited States is: a 5 l i V In a process of making a transformercomprising a core of magnetic material With superimposed input andoutput windings wound' on the core', thesteps compris- The bared wireend portion 140 extends ing: winding insulated wire around the; core .toform one of saidfwindings, applying-maskingftape longitudinally onthecore and .over said winding,tspraying the entire wound core with aconductive material which coversthe latter winding and said maskingtape,- removing said masking tape to leave an insulating gap extendingaround the. core perimeter, which gap prevents :the formation of a ashort circuit loop the coating of conductive material,

end portion and the conductive strip are isolated fromf direct electriccontact from the other longitudinal marginal portion of the-conductivecoating 1-24 bordering the insulating gap 126, to avoidproviding a shortcir-v cuitloop. v I 'j I t A layer 144 of insulation in the form of astrip of- Mylar material spirally wound around the core unit may then beapplied'around the core unit to insulatethe conductive stripll36 andmore importantly,to' protect'the windings to be tightly applied around,the exposed porapplying over the insulating-gap a strip of insulatingmaterial wider than said in-sulating gap so that the longie tudinalmargins thereof extend beyond the margins of the gap .on the outside'ofthe coating, applying a strip of conductive material over the :strip ofinsulatingmaterial which strip is narrower than the strip of insulatingma- 7 terial so; that the margins thereof terminate within the tions ofthe zinc coating 124 frorrr damag e by their contact with the roughsurface of the zinc coating. This insulating layer144 could be omittedwhere the' insulatioin r of the windings to be applied over, thevshielding construction just described is not damaged by the zinc coatingand is otherwise suitable as insulation. V y

. Next, the output windings 8b,18c and 8d arewound around the shieldingconstruction just described indifferent angular positions. around thecore-as shown. in

FIG. 8. Individual Mylar strips 146, 148 and l5 0 'of 'insulation arethen wound around the individual windings,

8b, 8c and 8d. The various leads extending to the windings of thetransformer unit are shown loosely extending fromthe transformer.However, these windings canivbe gathered together at any suitable pointor in a number of different points in a manner wellrknown in the art.

marginsof the strip of insulating material to provide an electricalshield over said gap,- and electrically connecting the'strip ofconductive material with only one of the longitudinal marginal portionsof said coating bordering said insulating gap to prevent, the formationof a short a circuit loop by placing a circumferentially extending bare3,063,135 1 1/62 Clark It should be further understood that additionalwinding" layers or shielding layers may be applied around orbetween thewindings illustrated in the drawings without deviating from the basicaspects of the invention.

The shielding construction above described can be ended insulatedconductor on only one'of the. contiguous portions of said strip ofconductive material and said coating andsoldering the bare end of saidconductor to said contiguousportions, and winding insulated wire aroundthe outside of said coated winding on the core wHrr oRa W'IL'TZ, PrimaryExaminer.

